Liquid crystal shutter and method of driving the same

ABSTRACT

A liquid crystal shutter comprises a liquid crystal device including a nematic liquid crystal sealed in between a first transparent substrate and a second transparent substrate on whose inner surfaces are formed respective transparent electrodes, the liquid crystal device having a twisted angle equal to or greater than 180°; and a pair of polarizing plates between which are sandwiched the first transparent substrate and the second transparent substrate, the polarizing films having respective absorption axes ( 13, 14 ) which are substantially orthogonal to each other, the absorption axes ( 13, 14 ) of the polarizing films being angled within a range of ±40° to ±50° relative to a direction ( 12 ) in which intermediate liquid crystal molecules are orientated, the direction indicating a direction of orientation of the liquid crystal in the intermediate portion in the direction of thickness of the liquid crystal device. Alternatively, Δ nd may lie within a range of 600 to 900 nm, Δ nd being the product of a birefringence Δ n of the nematic liquid crystal and a gap d between the first transparent substrate and a second transparent substrate.

INDUSTRIAL FIELD

The present invention relates to a liquid crystal shutter having a fastresponse such as a liquid crystal optical printer or a liquid crystaloptical device (which is used, for instance, in a color video printer oran LED combined field sequential color display), and to a method ofdriving the liquid crystal shutter.

BACKGROUND ART

Requirements for a liquid crystal shutter for use in a liquid crystalprinter or a liquid crystal optical device are a rapid response, abright display, a high contrast and a simple driving method, as well asa possible gradation display. However, a liquid crystal shuttersatisfying all these requirements has not been developed so far.

The liquid crystal shutters which have hitherto been developed areroughly grouped into the following three categories by liquid crystalmaterials used:

(1) one using a general nematic liquid crystal;

(2) one using a nematic liquid crystal for two-frequency driving methodhaving a positive or negative dielectric constant depending on thefrequencies; and

(3) one using a ferroelectric liquid crystal having a spontaneouspolarization.

The liquid crystal shutter using the two-frequency driving methodmentioned above (2) has a rapid response but has a complicated drivingcircuit due to its high driving voltage and high driving frequency.

The liquid crystal shutter using the ferroelectric liquid crystal of (3)above operates faster than that using the two-frequency driving liquidcrystal, that is, with a response time of several tens of μs, but isdeficient in the stability of orientation due to use of a smectic liquidcrystal phase. It also brings about a sticking phenomenon in which adisplay pattern remains fixed due to the DC drive and entails inprinciple a difficulty with the gradation control, which prevent it frombeing put to practical use except in certain specific applications.

The liquid crystal shutter using the general nematic liquid crystalmentioned above (1) employs the following systems depending on theprinciple of operation:

(a) a so-called TN (twisted nematic) liquid crystal system in which awhite or black display is performed by utilizing a phenomenon calledrotary polarization, rotating the incident light, in which a black orwhite display is performed by applying a voltage to pixels so as toorientate the liquid crystal molecules substantially orthogonal to thesubstrates to thereby eliminate the rotary polarization; and

(b) a so-called STN (super twisted nematic) liquid crystal system inwhich a white or black display is performed by utilizing birefringencecausing a phase difference in the incident light, in which a black orwhite display is performed by applying a voltage to the display pixelsto thereby vary the birefringence.

An example of the liquid crystal system of (a) above is found inJapanese Patent Laid-open Pub. No. Sho62-150330.

Reference is made to FIGS. 10 and 11 to explain this. FIG. 11 is aschematic sectional view of the conventional TN liquid crystal shutter,and FIG. 10 is a top plan view showing a relationship between absorptionaxes of polarizing plates and the direction in which liquid crystalmolecules are orientated, obtained when a liquid crystal shutter shownin FIG. 11 is viewed from the upper polarizing plate side.

As illustrated in FIG. 11, the liquid crystal device comprises a firsttransparent substrate 1 on which are formed a transparent firstelectrode 2 made of indium tin oxide (ITO) and an orientation film 3, asecond transparent substrate 4 on which are formed a transparent secondelectrode 5 made of ITO and an orientation film 6, and a nematic liquidcrystal 7 sealed in between the first and second substrates. On the topand bottom of the liquid crystal device there are arranged an upperpolarizing plate 9 and a lower polarizing plate 8, respectively, in sucha manner that their respective absorption axes are orthogonal to eachother, to thereby constitute the TN liquid crystal shutter.

As shown in FIG. 10, in this case, the liquid crystal device has atwisted angle of 90°, with the absorption axis 13 of the lowerpolarizing plate 8 being parallel to the direction 10 in which lowerliquid crystal molecules are orientated, that is, the direction oforientation of molecules closer to the first transparent substrate 1,and with the absorption axis 14 of the upper polarizing plate 9 beingparallel to the direction 11 in which upper liquid crystal molecules areorientated, that is, the direction of orientation of the liquid crystalcloser to the second transparent substrate 4.

With no voltage applied, in this TN liquid crystal shutter, linearlypolarized light transmitted through the lower polarizing plate 8 isrotated by 90° due to the rotary polarization of the liquid crystal andexits the upper-polarizing plate 9, resulting in an opened stateallowing a so-called positive display. When a 15V voltage is applied ata 5 kHz driving frequency between the first electrode 2 and the secondelectrode 5, the molecules of the nematic liquid crystal are orientatedin the direction orthogonal to the transparent substrates 1 and 4 tonullify the rotary polarization, thus allowing the linearly polarizedlight transmitted through the lower polarizing plate 8 to advanceintactly through the interior of the liquid crystal device without anyrotation and to be blocked by the upper polarizing plate 9, resulting ina closed state.

An example employing method (b) above includes an STN liquid crystaldisplay called a yellow mode for use in general liquid crystal displays.A conventional example thereof will be described with reference to FIGS.12 and 13.

FIG. 13 is a schematic sectional view of a conventional STN liquidcrystal display, and FIG. 12 is a top plan view showing a relationshipbetween the absorption axes of the polarizing films and the direction inwhich the liquid crystal molecules are orientated, obtained when FIG. 13is viewed from the upper polarizing plate side.

The configuration of the liquid crystal device shown in FIG. 13 issimilar to the configuration of the liquid crystal device shown in FIG.11, and hence identical parts to those of FIG. 11 are designated by thesame reference numerals and are not again described.

On the top and bottom of the liquid crystal device having the nematicliquid crystal 7 sealed in between the first and second transparentsubstrates 1 and 4 there are arranged the upper polarizing plate 9 andthe lower polarizing plate 8 in such a manner that their respectiveabsorption axes intersect at 60° relative to each other, therebyconstituting an STN liquid crystal display.

As shown in FIG. 12, in this case, the liquid crystal device has atwisted angle of 240°, with the absorption axis 13 of the lowerpolarizing plate 8 being angled at 45° relative to the direction 10 inwhich the lower liquid crystal molecules are orientated, that is, thedirection of orientation of the liquid crystal closer to the firsttransparent substrate 1, and with the absorption axis 14 of the upperpolarizing plate 9 being angled at 45° relative to the direction 11 inwhich the upper liquid crystal molecules are orientated, that is, thedirection of orientation of the liquid crystal closer to the secondtransparent substrate 4.

Thus, relative to the direction 12 in which the intermediate liquidcrystal molecules are orientated, that is, the direction of orientationof the liquid crystal molecules intermediate between the firsttransparent substrate 1 and the second transparent substrate 4, theabsorption axis 13 of the lower polarizing plate 8 forms an angle of 75°with the absorption axis 14 of the upper polarizing plate 9 forming anangle of 15°.

With no voltage applied, in this STN liquid crystal display, linearlypolarized light incident at 45° relative to the liquid crystal moleculesthrough the lower polarizing plate 8 is turned into ellipticallypolarized light due to the birefringence of the nematic liquid crystal7, which in turn exits the upper polarizing plate 9, resulting in anopened state allowing a yellowish white color display, that is, aso-called positive display. When a 3 to 5V voltage is applied at a 1 to5 kHz frequency between the first electrode 2 and the second electrode5, the molecules of the nematic liquid crystal 7 are orientated in thedirection orthogonal to the transparent substrates 1 and 4 to reduce itsbirefringence, thus allowing the linearly polarized light incidentthrough the lower polarizing plate 8 to undergo a varied state ofelliptical polarization, and in turn exits the upper polarizing plate 9in a bluish black display in the closed state.

In the case of system (a) above, however, the response time taken toreturn to the opened state by the removal of voltage from the closedstate is as long as ten to several tens of ms although the response timetaken to reach the closed state by the applying of voltage from theopened state is as short as several ms. Hence, when using it as theliquid crystal shutter for optical printers, the frame term must beincreased corresponding to a write term in which opening and closing arerepeated, resulting in an increased write time and a reduced printspeed. It is also impossible to apply it to a high-speed liquid crystaloptical device required to have a frame term of several ms.

Furthermore, the above publication teaches that the liquid crystaldevice having a 270° or 450° twist other than a 90° twist is morepreferred due to a reduction in the response time taken to recover theopen state.

Although it is certain that the 270° twist is shorter in response timethan the 90° twist, a specific orientation film, such as SiOorthorhombic deposited film, ensuring that a high pre-tilt must be usedwith a concurrent difficulty of obtaining satisfactory stability inorientation, which is not practical.

In the case of system (b) above, the liquid crystal device can be apractical so-called STN liquid crystal device having a 225° to 250°twist, thereby reducing the response time from the closed state to theopened state to several ms. As a result of application of voltage to theliquid crystal device, however, the closed state presents a bluish blackcolor and hence the contrast is as low as about 10. In addition, whenthe applied voltage is further raised, the state of the ellipticallypolarized light becomes changed, again allowing a brightening, so thatthe applied voltage cannot be set so high. It results in that theresponse time from the opened state to the closed state is increased toten to several tens of ms, making it difficult to use it as a liquidcrystal shutter.

It is therefore the object of the present invention to provide a liquidcrystal shutter ensuring a rapid response and a high contrast as well asa liquid crystal shutter driving method capable of a gradation display.

DISCLOSURE OF THE INVENTION

In order to achieve the above object, a liquid crystal shutter inaccordance with the present invention comprises: a liquid crystal deviceincluding a nematic liquid crystal sealed in between a first transparentsubstrate and a second transparent substrate on whose inner surfaces areformed respective transparent electrodes, the liquid crystal devicehaving a twisted angle equal to or greater than 180°; and a pair ofpolarizing plates between which are sandwiched the first transparentsubstrate and the second transparent substrate, the polarizing filmshaving respective absorption axes which are substantially orthogonal toeach other, the absorption axes of the polarizing films being angledwithin a range of ±40° to ±50° relative to a direction in whichintermediate liquid crystal molecules are orientated, the directionindicating a direction of orientation of the liquid crystal in anintermediate portion in a direction of thickness of the liquid crystaldevice.

Alternatively, when absorption axes of the polarizing films aresubstantially orthogonal to each other, Δ nd may lie within a range of600 to 900 nm, the Δ nd being the product of a birefringence Δ n of thenematic liquid crystal and a gap d between the first transparentsubstrate and the second transparent substrate.

More preferably, the polarizing films have respective absorption axeswhich are substantially orthogonal to each other, the absorption axes ofthe polarizing films being angled within a range of ±40° to ±50°relative to a direction in which intermediate liquid crystal moleculesare orientated, the direction indicating a direction of orientation ofthe liquid crystal in the intermediate portion in the direction ofthickness of the liquid crystal device, Δ nd lying within a range of 600to 900 nm, the Δ nd being the product of a birefringence Δ n of thenematic liquid crystal and a gap d between the first transparentsubstrate and the second transparent substrate.

In a method of driving a liquid crystal shutter in accordance with thepresent invention, a single drive term of the liquid crystal shutter isdivided into a reset term during which all pixels of the liquid crystalshutter are rendered closed and a scan term during which all the pixelsor predetermined pixels are rendered opened or half-opened; the durationof the scan term being made shorter than a holding time taken for atransmittance of the liquid crystal shutter to start to lower after ithas reached its maximum with no driving voltage applied to the liquidcrystal shutter.

A positive or negative driving voltage can be applied to the liquidcrystal shutter during a partial period within the scan term, thedriving voltage being set to 0V during the remaining period, the periodduring which the driving voltage is set to 0V being varied to perform agradation display.

The voltage applied to the liquid crystal shutter in the scan term maybe varied from 0V to perform a gradation display.

Preferably, a single driving term of the liquid crystal shutter iscontrolled, depending on the operating temperature, so as to beincreased at the time of a low temperature but reduced at the time of ahigh temperature.

A single driving term of the liquid crystal shutter may be assignedexclusively to a scan term during which all pixels or predeterminedpixels of the liquid crystal shutter are rendered opened or half-opened,the scan term being controlled, depending on the operating temperature,so as to be lengthened at the time of a low temperature but shortened atthe time of a high temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view showing a relationship between absorption axesof polarizing plates and the direction in which liquid crystal moleculesare orientated, obtained when a liquid crystal shutter shown in FIG. 2is viewed from its upper polarizing plate side;

FIG. 2 is a schematic sectional view of the liquid crystal shutter whichis an embodiment of the present invention;

FIG. 3 illustrates for comparison purposes transmittance-voltage curvesshowing variance of the transmittance relative to applied voltage, ofthe liquid crystal shutter in accordance with the present invention andof the conventional liquid crystal shutter;

FIG. 4 is a diagram showing a relationship between the arrangement angleof the polarizing plate and the transmittance in the liquid crystalshutter in accordance with the present invention;

FIG. 5 is a diagram showing a relationship between Δ nd of the liquidcrystal device and the transmittance in the liquid crystal shutter inaccordance with the present invention;

FIG. 6 is a diagram showing a driving waveform and a transmittance-timecurve for the purpose of explaining a fundamental method of driving theliquid crystal shutter in accordance with the present invention;

FIG. 7 is a diagram showing a driving waveform and a transmittance-timecurve of the liquid crystal shutter in accordance with the presentinvention, obtained when the liquid crystal shutter is applied to acolor video liquid crystal printer;

FIG. 8 is also a diagram showing a driving waveform and atransmittance-time curve of the liquid crystal shutter at roomtemperature, for the purpose of explaining another driving method;

FIG. 9 is also a diagram showing a driving waveform and atransmittance-time curve of the liquid crystal shutter at a lowertemperature;

FIG. 10 is a top plan view showing a relationship between absorptionaxes of polarizing plates and the direction in which liquid crystalmolecules are orientated, obtained when a conventional liquid crystalshutter shown in FIG. 11 is viewed from its upper polarizing plate side;

FIG. 11 is a schematic sectional view showing by way of example aconventional liquid crystal shutter;

FIG. 12 is a top plan view showing a relationship between absorptionaxes of polarizing plates and the direction in which liquid crystalmolecules are orientated, obtained when a conventional STN liquidcrystal display shown in FIG. 13 is viewed from its upper polarizingplate side; and

FIG. 13 is a schematic sectional view showing an example of theconventional STN liquid crystal display.

BEST MODE FOR CARRYING OUT THE INVENTION

In order to describe the present invention in more detail, a mostpreferred embodiment of the invention will now be described withreference to the accompanying drawings.

Liquid Crystal Shutter in Accordance With the Present Invention

FIG. 2 is a schematic sectional view showing a structure of a liquidcrystal shutter which is an embodiment of the present invention, andFIG. 1 is a top plan view showing a relationship between absorption axesof polarizing plates and directions in which liquid crystal moleculesare orientated, when the liquid crystal shutter of FIG. 2 is viewed fromthe upper polarizing plate side. In these diagrams, parts correspondingto those of FIGS. 10 to 13 illustrating the above conventional examplesare designated by the same reference numerals.

Referring first to FIG. 2, this liquid crystal shutter comprises a firsttransparent substrate 1 made of 0.7 mm-thick glass on which are formed afirst transparent electrode 2 made of ITO and an orientation film 3; anda second transparent substrate 4 made of 0.7 mm-thick glass on which areformed a second electrode 5 made of ITO and an orientation film 6, witha nematic liquid crystal 7 being sealed in between the first and secondsubstrates to constitute a liquid crystal device. The birefringence Δ nof the nematic liquid crystal used in this shutter is 0.2, the gap dbetween the first transparent substrate 1 and the second transparentsubstrate 4 is 4 μm, and the value of Δ nd indicating a birefringence asa liquid crystal device is set at 800 nm.

The orientation film 3 above the first transparent substrate 1 ispreviously subjected to a rubbing treatment in the direction 10 in whichlower liquid crystal molecules are orientated as illustrated in FIG. 1.The orientation film 6 above the second transparent substrate 4 ispreviously subjected to the rubbing treatment in the direction 11 inwhich upper liquid crystal molecules are orientated as illustrated inFIG. 1. To twist liquid crystal molecules, a chiral material is added tothe nematic liquid crystal 7 having a viscosity of 18 cp, allowing itsnatural twist pitch to be 8 μm, to thereby form a leftward 240° twistedliquid crystal device.

On the top and bottom of the liquid crystal device there are arranged anupper polarizing plate 9 and a lower polarizing plate 8, respectively,in such a manner that their respective absorption axes 14 and 13 areorthogonal to each other.

At that time, the absorption axis 13 of the lower polarizing plate 8 isangled at 45° counterclockwise relative to the direction 12 in whichintermediate liquid crystal molecules are orientated, the direction 12indicating the direction of orientation of the intermediate portion ofthe nematic liquid crystal 7 between the first transparent substrate 1and the second transparent substrate 4, whereas the absorption axis 14of the upper polarizing plate 9 is angled at 45° clockwise relative tothe direction 12 in which the intermediate liquid crystal molecules areorientated, thus constituting a positive liquid crystal shutter.

In the state where no voltage is applied to this liquid crystal shutter,linearly polarized light that has passed through the lower polarizingplate 8 is turned by the birefringence of the liquid crystal into anelliptically polarized light, which is then allowed to exit the upperpolarizing plate 9 to form a slightly yellowed white display at anopened state, or a so-called positive display.

When a DC or AC voltage of 10 to 20V is applied between the firstelectrode 2 and the second electrode 5, the molecules of the nematicliquid crystal 7 are orientated in the direction orthogonal to thetransparent substrates 1 and 4 so that the birefringence is nullified,allowing linearly polarized light which has passed through the lowerpolarizing plate 8 to travel intactly through the interior of the liquidcrystal device to be blocked by the upper polarizing plate 9, thusforming a black display at a closed state.

Characteristics of the Above Liquid Crystal Shutter

Reference is now made to FIGS. 3 to 5 to describe the characteristics ofthis liquid crystal shutter.

A solid line 20 in FIG. 3 represents a transmittance-voltage curve ofthe liquid crystal shutter set forth hereinabove (a broken line 21represents a conventional one). Starting from the initial transmittanceY0 with no voltage applied, the transmittance gradually risesaccordingly as more voltage is applied between the first and secondelectrodes 2 and 5, and reaches a maximum transmittance Ym in thevicinity of an applied voltage of 2V. When the applied voltage isfurther increased, the transmittance lowers. The transmittance at anapplied voltage of 10V becomes about a fiftieth of the initialtransmittance Y0, with a contrast ratio on the order of 50. Applicationof a voltage higher than 20V results in the acquisition of a contrastratio of more than 100.

Since the opened state allowing the white display with no voltageapplied is presented by making use of the birefringence of the liquidcrystal device as described above, the arrangement of the polarizingfilms 8 and 9 and the setting of Δ nd indicative of the birefringence ofthe liquid crystal device are essential, which affect the brightness andthe colored state to a large extent.

FIG. 4 shows arrangement angles of the lower polarizing plate 8 and thetransmittance of the liquid crystal shutter, obtained when the lowerpolarizing plate 8 is rotated counterclockwise from the direction 12 inwhich the intermediate liquid crystal molecules are orientated, with theintersection angle fixed at 90° between the absorption axis 13 of thelower polarizing plate 8 and the absorption axis 14 of the upperpolarizing plate 9 in a 240° twisted liquid crystal device having avalue of Δ nd equal to 800 nm.

A solid line 22 represents a relationship between the maximumtransmittance Ym and the arrangement angle of the polarizing plate,while a broken line 23 represents a relationship between the initialtransmittance Y0, with no voltage applied, and the arrangement angle ofthe polarizing plate.

At ˜60°, the direction 10 in which the lower liquid crystal moleculesare orientated becomes parallel to the absorption axis 13 of the lowerpolarizing plate 8. It is most advantageous at −45° and +45° in thatboth Y0 and Ym present their respective local maximum values with lesscolored conditions.

FIG. 5 shows Δ nd of the 240° twisted liquid crystal device and thetransmittance of the liquid crystal shutter, obtained when theabsorption axis 13 of the lower polarizing plate 8 is positioned at 45°counterclockwise from the direction 12 in which the intermediate liquidcrystal molecules are orientated, with an intersection angle of 90°between the absorption axis 13 of the lower polarizing plate 8 and theabsorption axis 14 of the upper polarizing plate 9.

A solid line 24 represents the maximum transmittance Ym and a brokenline 25 represents the initial transmittance Y0 at the time of noapplied voltage. At Δ nd=650 nm, the maximum transmittance Ym reachesits maximum and thereafter remains substantially unvaried even though Δnd further increases, whereas the initial transmittance Y0 with noapplied voltage gradually lowers, so that it is not preferred for Δ ndto become too large. On the contrary, when Δ nd is smaller than 650 nm,the maximum transmittance Ym also decreases, with the result that apreferred value of Δ nd lies within a range of 600 nm to 900 nm, andparticularly 700 nm to 800 nm.

Although it may vary more or less depending on the twisted angle, theoptimum value of Δ nd lies within a range of about 600 nm to 900 nm, inthe case of a twisted angle of 180° to 260°.

In this embodiment, Δ nd is set at 800 nm with the twisted angle of240°, so that the opened state ensuring a bright and relatively lesscolored white display is achieved with a contrast of more than 100 uponthe application of a drive voltage of 20V.

Method of Driving the Liquid Crystal Shutter

Description will next be made of a method of driving the liquid crystalshutter set forth hereinabove.

FIG. 6 is a diagram depicting a driving waveform 30 and atransmittance-time curve 31 representative of the variance with time ofthe transmittance, obtained when a 100 Hz, 20V AC signal is applied for50 ms between the first and second electrodes 2 and 5 of the liquidcrystal shutter shown in FIGS. 1 and 2.

When the AC signal is applied under the opened state (white display) atno applied voltage, as can be seen from this diagram, the transmittanceinstantaneously rises and thereafter sharply drops resulting in theclosed state (black display). An on-response time 26 at that time isinfluenced by the applied voltage so that accordingly as a highervoltage is applied to the liquid crystal shutter, the on-response time26 decreases. In this embodiment, the liquid crystal shutter issubjected to a high voltage of 20V, thus achieving a very rapidon-response time 26 of less than one ms.

On the contrary, when the AC signal is returned to 0V under the closedstate, the transmittance reaches its maximum in about two ms andrecovers its initial value after an elapse of about 20 ms. Due to theutilization of a resilient force untwisting the liquid crystal twist, anoff-response time for returning from the closed state to the openedstate decreases accordingly as the twisted angle of the liquid crystaldevice increases. The proper definition of the response time for theliquid crystal device is the time taken for the variance of the liquidcrystal molecules to become stabilized. Hence, in FIG. 6, the responsetime is 20 ms. In the case of being used as the liquid crystal shutter,however, the time taken to return to the opened state allowing the whitedisplay is effective as the response time. Thus, the off-response time27 of the 240° twisted liquid crystal shutter in accordance with thepresent invention is 2 ms, achieving a rapid response liquid crystalshutter.

Until the maximum transmittance in the opened state is exhibited afterthe black display in the closed state, there is provided a relativelyless colored and bluish white display. After the elapse of about 10 msof a holding time 28 during which its maximum transmittance is kept, thetransmittance lowers with the white display yellowed to some extent. Inorder to execute a gradation display, therefore, a reset signal isissued within the holding time 28 during which the liquid crystalshutter exhibits its maximum transmittance, to restore the closed state.Thus, by utilizing the less colored state between the closed state andthe maximum transmittance, a satisfactory gradation display is achieved.

FIG. 7 shows a driving waveform 32 and a transmittance-time curve 33,obtained when the liquid crystal shutter in accordance with the presentinvention is applied to a color video liquid crystal printer.

In FIG. 7, Tf represents a frame term corresponding to a single writeterm, which consists of a reset term Tr and a scan term Ts. The resetterm Tr is set to be 1 ms which is longer than the on-response time 26shown in FIG. 6 and the scan term Ts is set to be 4 ms which is shorterthan the 10 ms of the holding time 28.

A first frame at the left-hand end of FIG. 7 indicates a fully-openedstate, a second frame in the middle indicates a half-opened state and athird frame at the right-hand end indicates a closed state.

In the reset term Tr, to render all pixels of the liquid crystal shutterclosed, a 20V DC signal is applied as a reset signal to all the pixels.

In the case of rendering the liquid crystal shutter fully opened in thescan term Ts, a 0V data signal is applied over the entire duration ofthe scan term Ts. In the scan term Ts as well, when keeping the liquidcrystal shutter closed, a 20V data signal is applied over the entireduration of the scan term Ts. In the case of making the liquid crystalshutter half-closed to present halftones in the scan term Ts, a 0V datasignal is applied for 2 ms equal to half of the scan term Ts and a 20Vdata signal is applied for the remaining 2 ms.

By inverting polarities of the reset signal and data signal from frameto frame, long-term DC voltage application to the liquid crystal deviceis suppressed. After the return of all the pixels of the liquid crystalshutter to their respective closed states in the reset term Tr, a periodof time during which a 0V data signal is applied can be varied in thescan term Ts so that only a predetermined pixel is opened or closed andan arbitrary gradation display is obtained.

The scan term Ts is set at 4 ms, which is longer than the 2 ms of theoff-response time 27 taken to reach the maximum transmittance Ym afterthe closed state shown in FIG. 6 and is shorter than the 10 ms of theholding time 28 taken to start to return to the initial transmittance Y0from the maximum transmittance Ym, so that it is possible to obtain agradation display subjected to less variation in color and having a goodlinearity, thus achieving the acquisition of a high-quality full colorimage print.

Although this embodiment employs the 240° twisted liquid crystal device,any other liquid crystal device having a twist of more than 180° is alsoapplicable to obtain similar effects.

It is sufficient if the absorption axes of the upper and lowerpolarizing plates 9 and 8 intersect at approximately 90° and that thearrangement angles of the polarizing films lie within a range of 40° to50° relative to the direction in which the intermediate liquid crystalmolecules are orientated.

Alternatively, the intersecting angle of the absorption axes of theupper and lower polarizing plates 9 and 8 may be reduced to 80° to 85°to further improve the coloring at the time of no applied voltage.

Although in this embodiment the absorption axes of the polarizing filmsare angled at ±45° relative to the direction in which the intermediateliquid crystal molecules are orientated and Δ nd of the liquid crystaldevice is set at 800 nm, a certain effect is achieved merely byemploying the ±45° arrangement of the polarizing films or by setting Δnd of the liquid crystal device to 600 to 900 nm.

Although in this embodiment such a description has been made that thegradation control is performed by varying the period of time duringwhich the 0V scan signal is issued in the scan term Ts, the gradationcontrol may be carried out by changing the voltage applied in the scanterm from 0V so as to increase the off-response time.

Another embodiment of a method of driving the liquid crystal shutter inaccordance with the present invention will now be described withreference to FIGS. 8 and 9.

FIG. 8 is a diagram depicting a driving waveform 34 and atransmittance-time curve 35 at room temperature, obtained when theliquid crystal shutter of the present invention is applied to a colorvideo liquid crystal printer, and FIG. 9 is a diagram depicting adriving waveform 36 and a transmittance-time curve 37 at 0° C.

In FIG. 8, the reset term Tr at room temperature (approx. 25° C.) is setat 1 ms and the scan term Ts is set at 4 ms which is shorter than the 10ms of the holding time 28 (FIG. 6) taken for the transmittance to startto lower from the maximum transmittance Ym.

A frame term Tf corresponds to a single write term, which consists of areset term Tr and a scan term Ts. In FIG. 8, a first frame at theleft-hand end is in a fully opened state, the second frame adjacent onthe right-hand side is in a half-opened state and the third frame whichfollows is in a closed state, with the first to third frames appearingthereafter in the same order.

In the reset term Tr at room temperature, to render all pixels of theliquid crystal shutter closed, positive and negative 20V, 0.5 ms-longpulse waveforms are applied as reset signals in pairs to all the pixels.Data signals applied in the scan term Ts are set at 0V throughout thescan term Ts in the case of making the liquid crystal shutter fullyopened. When keeping it closed, they are fed in the form of 20V, 0.5ms-long pulse waveform signals throughout the scan term Ts. In the caseof rendering it half-opened to present halftones, they are set at 0V for2 ms equal to half of the scan term Ts but fed in the form of 20V, 0.5ms-long pulse waveform signals for the remaining 2 ms of the term.

In this driving method, the reset signals and data signals are issued inthe form of 0.5 ms pulse waveforms having positive and negativepolarities so as to suppress long-term DC voltage application to theliquid crystal device. After the return of all the pixels of the liquidcrystal shutter to their respective closed states in the reset term Tr,a period of time during which 0V data signals are issued can be variedin the scan term Ts so that only a predetermined pixel is opened orclosed and an arbitrary gradation display is obtained.

The scan term Ts at room temperature is set at 4 ms, which is longerthan the 2 ms of the off-response time taken to reach the maximumtransmittance Ym from the closed state and shorter than the 10 ms of theholding time taken for the transmittance to return to the initialtransmittance Y0 from the maximum transmittance Ym, so that it ispossible to obtain a gradation display subjected to less variation incolor and having a good linearity.

A reduction in the temperature, however, results in an increasedresponse time of the liquid crystal device. Since in particular theoff-response time from the closed state to the opened state increases,the brightness in the opened state is reduced, so that no opened stateis exhibited at a further lowered temperature.

In this embodiment of the driving method, therefore, a temperaturesensor is provided which, when the temperature goes down to 5° C. orbelow, automatically doubles the reset time Tr and the scan time Ts.

Regarding the response time at 0° C. of the liquid crystal shutter inaccordance with the present invention, as is apparent from thetransmittance-time curve 37 at 0° C. shown in FIG. 9, the on-responsetime from open to close is 1.5 ms and the off-response time from closeto open is 4 ms, which are approximately twice as much as the respectiveresponse times at room temperature.

At 0° C., the holding time also approximately doubles, namely, increasesto 20 ms. As seen from the waveform 36 of FIG. 9, the drive waveform at0° C. has a 2 ms reset term Tr and an 8 ms scan term Ts, which are twiceas much as the respective terms at room temperature, thereby ensuring asatisfactory opened state.

In the case where this liquid crystal shutter driving method is appliedto a liquid crystal printer, a high-quality full color image print canbe obtained both at room temperature and 0° C. although the print speedat a low temperature is reduced to half of the print speed at roomtemperature.

Although in this driving method, the frame term Tf consisting of thereset term Tr and the scan term Ts has been increased twice as muchwithout varying the pulse length even at a low temperature, the pulselength may be simultaneously doubled to obtain exactly the same effect.

In the case of not needing the halftone display, the frame term Tf maybe composed of only the scan term Ts without the reset term Tr, insteadof composing the frame term Tf of both the reset term Tr and the scanterm Ts.

Although in the above driving method, such description has been madethat the gradation control is carried out by varying the period of timeduring which a 0V scan signal is issued in the scan term Ts, thegradation control may be performed by changing the voltage applied inthe scan term Ts from 0V to thereby increase the off-response time.

INDUSTRIAL APPLICABILITY

As is apparent from the above description, the liquid crystal shutter inaccordance with the present invention and the execution of the method ofdriving the liquid crystal shutter ensures a realization of a rapidresponse of the liquid crystal shutter and a bright, high contrast,while allowing a gradation display.

Stabilized shutter characteristics can also be maintained even thoughthe liquid crystal shutter is operated at a wide-ranging operatingtemperature from a low temperature to a high temperature.

Thus, application of the present invention to the liquid crystal shutterof a color video liquid crystal printer and to the method of driving thesame results in a constant acquisition of a high-quality full colorimage print.

The liquid crystal shutter and the method of driving the same inaccordance with the present invention are applicable otherwise to, e.g.,a field sequential color display, which is a combination of lightemitting diodes (LEDs) and the liquid crystal shutter.

What is claimed is:
 1. A liquid crystal shutter comprising: A liquidcrystal device including a nematic liquid crystal sealed in between afirst transparent substrate and a second transparent substrate on whoseinner surfaces are formed respective transparent electrodes, said liquidcrystal device having a twisted angle of greater than 180° and less thanor equal to 260°; and a pair of polarizing plates between which aresandwiched said first transparent substrate and said second transparentsubstrate, said polarizing plates having respective absorption axeswhich are orthogonal to each other, said absorption axes of saidpolarizing plates being angled with a range of ±40° to ±50° relative toa direction in which intermediate liquid crystal molecules areorientated, said direction indicating a direction of orientation of saidliquid crystal in an intermediate portion in a direction of thickness ofsaid liquid crystal device; wherein said liquid crystal device performswhite display utilizing birefringence of said liquid crystal whenvoltage is not applied thereto, and performs black display when drivenby applying DC or AC voltage of 10 to 20V, and birefringence of saidliquid crystal device is nullified when said voltage is applied to saidliquid crystal device.
 2. A liquid crystal shutter according to claim 1,wherein Δ nd lies within a range of 600 to 900 nm, said Δ nd being theproduct of a birefringence Δ n of said nematic liquid crystal and a gapd between said first transparent substrate and said second transparentsubstrate.